This invention relates to laser printers and, more particularly, to high resolution or typographical quality laser printers.
In recent years, laser printers have been utilized in increasing numbers as a means for producing hard copy from a computer, computer generated video or video source. The more advanced of these laser printers produce the hard copy with good resolution in a relatively short period of time.
Basically, a laser printer uses an optical lens system to focus the light beam output of a laser to a spot on a photosensitive surface. The spot is caused to sweep across the surface in a raster fashion by a rotating or oscillating mirror. Computer or video raster information modulates the light beam as the photosensitive surface moves in a direction perpendicular to and under the sweeping light beam. If the photosensitive surface is the photoconductor drum in an electrophotographic process, the sweeping and modulated light beam causes the formation of an electrostatic charge image on the drum. This image can then be developed, transferred to paper and fused with the paper in a manner typical of electrophotographic copiers. An example of such a laser printer is disclosed in U.S. Pat. No. 3,867,571 of Starkweather, et al.
Laser printers typically have resolutions of about 240 dots/inch and most have resolutions under 400 dots/inch. Thus, they are generally limited to computer data output and general office use. In the printing and publishing industries, resolutions in excess of 500 dots/inch are used. To achieve this resolution, electronic phototypesetters are generally used. An example of these devices is the Monotype Lasercomp manufactured by Monotype Corporation, Ltd. of Salfords, Redhill, England. Such phototypesetters use focused light, e.g. from a laser beam, to expose a photographic film medium and generally function like the system described in U.S. Pat. No. 4,257,053 of Gilbreath.
The difficulty with the electronic phototypsetters that expose film is that they are relatively slow, film is relatively expensive (well over 12 times the cost of paper and toner) and generally requires inconvenient, time-consuming chemical development.
There is a need in the publishing and printing industry for a device with the time, cost and convenience benefits of a laser printer and with sufficient resolution that it can be used to produce: (1) typographic page proofs of the image information which is intended to produce a copy or master from a phototypesetter; (2) direct output of typographic quality documents, i.e. demand printing; and (3) typographic quality masters for volume duplication. In order to achieve this, the resolution of laser printers must be significantly increased, without substantial sacrifice in the cost of the system.
If a high resoution is achieved, it can be lost due to variations in the corona charge placed on the photoconductor as a result of environmental factors. It is proposed in U.S. Pat. No. 3,604,925 of Snelling to correct for this by sensing the corona current via a wire located across the photoconductor drum and to use this signal to control the corona voltage to keep the sensed current constant. This is a relatively sensitive arrangement, however, and it increases the cost of the system because of the need for the sensing wire and its support structure.